(1) Field of the Invention
The invention relates to a device for controlling the fuel-air-ratio during combustion of ground coal in a firing system of a coal-fired power plant, which comprises means for pneumatic delivery of the ground coal to the burners of the coal-fired power plant and means for feeding the combustion air to the burners or the combustion chamber of the firing system of the coal-fired power plant, wherein at least the following devices are arranged in the flow direction of the air: a fresh air blower for suctioning fresh air from the atmosphere, a mill blower for transporting a portion of the suctioned fresh air as carrier air for loading with ground coal, a regenerative air preheater for preheating the suctioned fresh air and a portion of the carrier air by using the flue-gas heat from the firing system of the coal-fired power plant, wherein a storage mass of the regenerative air preheater is alternatingly heated by the hot flue gas and subsequently cooled by the fresh air or by a portion of the carrier air, an air quantity control device for controlling the quantity of combustion air to be used for the pneumatic transport of the ground coal, and measurement devices for measuring the quantity of combustion air introduced into the combustion chamber and of the quantity of carrier air to be used for pneumatic transport of the ground coal, and a device for metered delivery of a preselected quantity of ground coal to the burners. The invention further includes a method for controlling the fuel-a ratio during the combustion of ground coal in a firing system of a coal-fired power plant, which has at least the aforementioned features.
(2) Description of Related Art
The control of the fuel-air ratio during the combustion of ground coal firing systems of coal-fired power plants is particularly important for achieving a substantially complete combustion of the supplied fuel and/or for maintaining a predefined stoichiometry of the combustion process and hence for attaining a high energy efficiency and maintaining low emission values. Firing systems in coal-fired power plants therefore have control devices in addition to suitable devices for metered delivery of a fuel quantity which is preselected in accordance with the load demand, wherein the control devices control the quantity of combustion air supplied to the burners and/or the firing chamber as a function of the supplied fuel quantity. This requires measurement devices capable of measuring the quantity of combustion air introduced into the firing chamber as accurately as possible. Both are necessary for optimally controlling the combustion process depending on the load, wherein the fuel quantity is preselected in accordance with the load demand and the quantity of combustion air is controlled for attaining a predetermined stoichiometry of the combustion. In practice, on one hand, coal-fired power plants exist where a preselected quantity of ground coal is supplied to each individual burner or to a group of burners and the quantity of combustion air supplied to this burner or to this group of burners is controlled commensurate with this preselected quantity of ground coal for obtaining a predetermined stoichiometry of the combustion, and on the other hand, firing systems exist wherein only the total quantity of ground coal supplied to all burners of a boiler room is preselected and the quantity of combustion air supplied to all burners of this boiler or to the overall boiler is controlled commensurately.
It is necessary in any event to measure the quantity of combustion air either in relation to a single burner or a group of burners or all burners of a boiler or the entire boiler, in order control the quantity of combustion air and hence also the combustion process.
In addition, in firing systems of coal-fired power plants with pneumatic transport of the ground coal to the burners, the quantity of carrier air for the pneumatic coal transport is controlled. This control also requires a measurement of the quantity of carrier air.
The quantity of air in firing systems of coal-fired power plants is measured almost exclusively by differential pressure measurements using pressure sensors. To this end, pressure sensors are installed in the respective pipe or channel system carrying the combustion air, as well as in the pipe or channel system carrying the carrier air. Based on the measured pressures, the flow velocity in the channels can be determined and the quantity of air can be determined by taking into account the channel geometry. Preferably, the pressure sensors are not installed directly in the channel cross-section, but are connected with the channels carrying the air via so-called impulse pipes.
For improving the energy efficiency of a firing system of a power plant, the fresh air is typically preheated by alternatingly first heating a storage mass, mostly smooth or corrugated metal sheets, of a regenerative air preheater with hot flue gas and subsequently cooling the storage mass with the fresh air, thereby transferring heat from the flue gas to the fresh air. This is accompanied by the introduction of fly ash particles into the fresh air. During operation of a power plant firing system, this regularly causes contamination of the pressure sensors and/or the impulse pipes, requiring continuous cleaning and maintenance. The problem is that the degree of the contamination of the pressure sensors cannot be uniquely determined based on the measured difference pressures and that the measurement results can therefore have a significant error probability under continuing operation. This causes a drift in the air quantity measurement which is very difficult to detect. In the end, this causes an inaccurate control of the fuel-air ratio, accompanied with a reduced efficiency and increased emission of pollutants.